The present invention relates generally to a level shifter and a method for operating the same.
Logic device supply voltages are commonly approx. 1 V, that is a low voltage level. However, higher voltage levels are required for operating non-volatile flash memories, e.g. 10 to 17 V. Special high voltage (HV) devices are used in the logic process to handle these high voltages.
In general, a low voltage has a voltage level that is above 0 V. Further, a medium voltage has a voltage level that is above the voltage level of the low voltage, and a high voltage has a voltage level that is above the voltage level of the medium voltage. Specific transition values between the voltage levels of the low voltage, medium voltage and high voltage depend on the specific application of the logic device and are known in the arts.
To switch high output voltage signals 306, 308 by lower input voltage signals 310, 312 a so called level shifter is used. A known design of a level shifter uses a cascode structure using cascode terminal 314, 316—illustrated in FIG. 3—if the output high voltage is higher than the maximum allowed device operating voltage. The cascode structure is used to handle voltages that are higher than the allowed operating voltage of a device, e.g. a transistor, that are provided by supply voltage terminals 302, 304. In the cascode 318, a second transistor (source) is connected to the drain of a first transistor to reduce the overall voltage like a series resistance. Thus, the cascode circuit can operate higher voltages that the single transistors allow.
As in all other types of cascoded circuits, the circuit area size increases significantly. That is, larger silicon area consumption is required as the number of required transistors increases and the transistors width needs to be increased. In particular, the transistor layout of HV transistors is significantly larger than of low voltage (LV) transistors. Further, the manufacturing effort is increasing with increasing high voltage capabilities of the HV devices. For that reason, it would be preferred to implement the cheapest possible HV device to reduce process cost.